Chinese president Xi Jinping is widely considered to be China’s most authoritarian premier in decades.
Photograph: Xinhua/Barcroft Media

The family and friends of a Swedish human rights campaigner who appears to be the first foreigner to become entangled in a Communist party crackdown on Chinese civil society are demanding his release from Beijing.

Peter Dahlin, 35, disappeared on the night of 3 January while making his way to Beijing’s international airport, from where he had planned to fly to Thailand, colleagues say.

On Wednesday afternoon, the Chinese government confirmed Dahlin was in custody and said the Beijing-based activist was being held on suspicion of “endangering state security”.

The Swedish activist’s Chinese girlfriend has also reportedly disappeared.

Dahlin cofounded and ran an organisation called Chinese Urgent Action Working Group (CUAWG) which describes itself as “a team of human rights defenders working in mainland China … to assist fellow human rights defenders in distress”.

“The group works to protect human rights defenders in peril and facing political persecution for their work,” its website says.

Scores of activists have been jailed, detained and interrogated since a major offensive against human rights lawyers began last July.

In a statement, CUAWG claimed Dahlin, who has worked in China since 2007, had been “arbitrarily detained on spurious accusations”. It suggested the activist had fallen victim to China’s “six-month long assault on … human rights lawyers”.

Chinese claims that Dahlin had endangered state security were baseless, the group added.

“Peter’s ongoing detention for supporting legal aid in China makes a mockery of president Xi Jinping’s stated commitments to the rule of law,” said Michael Caster, a spokesman for the group. “The Chinese authorities must immediately release Peter from detention and drop all charges against him.”

A spokesperson for China’s foreign ministry confirmed they had taken “coercive measures” against Dahlin on Wednesday, though they had denied knowledge of the case the previous afternoon.

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China safeguarded the legal rights of foreigners, the spokesperson added.

“That is our biggest concern,” Jonas Dahlin, his brother, told the Guardian in an email on Wednesday. “Without his medicine he will most probably die.” The Swedish foreign ministry had been told that Peter Dahlin was receiving medicine, his brother said.

“[We have] no news on his whereabouts,” added Jonas, who is based in Thailand.

Caster attacked China’s “appalling” treatment of his colleague and claimed authorities had denied Dahlin access to Swedish officials in “direct violation of Chinese law and the Vienna convention on consular relations”.

Beijing has faced growing international criticism over the campaign its security services are waging against human rights lawyers.

However, Dahlin is the first foreigner to become embroiled in the crackdown and his detention will send a chill through Beijing’s already edgy NGO community.

Joshua Rosenzweig, a Hong Kong-based human rights expert, said Dahlin’s detention signalled that, having attacked domestic civil society, Beijing was now setting its sights on foreign individuals or groups it believed were attempting to “undermine the one-party system”.

“Whereas maybe five years ago someone would have been kicked out of China or not been allowed to return, arresting somebody sends a different message. It moves the line in terms of showing what steps the state is willing to take to reassert its control and to eliminate those it sees as threats to its control,” he said.

“It is going to raise a red flag for all foreigners and all foreign entities who are at all interested in trying to engage with Chinese social actors to try to build capacity for the development of civil society and for the development of rule of law,” he added.

Paramilitary guards stand in front of the gates of Sweden’s embassy in Beijing on Wednesday. Photograph: Fred Dufour/AFP/Getty Images

In an email to the Guardian on Wednesday, Sebastian Magnusson, a spokesman for the Swedish embassy in Beijing, wrote: “We know that a Swedish citizen, a man in his mid-30s, has been detained in China. The embassy is investigating this and has requested to meet him. We cannot provide any other information at this point.”

Speaking on Wednesday, Caster said: “The government is likely trying to send a signal with Peter’s detention. The question now is what that signal will be exactly and how it will end.

“With the size and severity of the crackdown on lawyers and their assistants it only makes sense from a totalitarian perspective to go after anyone who has provided them support or is actively sympathetic to their cause of promoting human rights. It is a startling reminder of what Xi Jinping’s China looks like.”